Mg-phosphate ceramics produced from the product of thermal transformation of cement-asbestos

According to recent European directives, the need for environmentally friendly alternative solutions to landfill disposal of hazardous wastes, such as asbestos-containing materials, prompts their recycling as secondary raw materials. In this work, magnesium phosphate ceramics were prepared using the product of inertization of cement-asbestos. Magnesium phosphate ceramics show interesting properties like good water resistance and high strength that make them attractive materials for several applications. Asbestos containing materials were mixed with magnesium carbonate and annealed at two different temperatures (1100 and 1300 °C). During thermal treatment complete destruction of asbestos minerals with their transformation into new phases, and crystallization of MgO from magnesium carbonate decomposition, occurs. Upon addition of potassium di-hydrogen phosphate and water, the magnesium oxide in the product of thermal treatment, contributes to the onset of a setting reaction whose product is magnesium potassium phosphate hydrate. The reactivity of periclase was found to be dependent on the calcination temperature. Lower reaction rates were observed for the MgO obtained at lower temperature. The setting reaction of the magnesium phosphate ceramic has been followed in time up to 6 months by means of the X-rays powder diffraction trechnique. Quantitative phase analysis was performed using the Rietveld method and both crystalline and amorphous phases were quantified. The amount of magnesium potassium phosphate was found to increase with time, and was accompained by a decrease in the amount of the amorphous fraction. This fact supports the hypothesis of the formation of an amorphous precursor of the crystalline MKP during the hydration reaction. SEM images showing elongated magnesium potassium phosphate hydrate crystals emerging from what appears as an amorphous matrix, further confirms this view. Since the mechanical properties of magnesium phosphate ceramics are known to increase with time, we can conclude that the main contribution to the development of strenght comes from the crystalline magnesium potassium phosphate hydrate. In this work, we describe a procedure for simultaneous destruction of asbestos minerals and formation of cementitious compounds, which represents a recycling opportunity for this class of hazardous wastes, bringing benefits in terms of energy requirements and preservation of natural resources in cement manufacturing.